Why are the internal parts of the model disappear in blender game engine
There are a few things that could be causing this.

First, backface culling

That happens because the inside of the mesh are backfaces and somewhere, (in the object data panel or in the materials panel in the properties menu) there is a checkbox for backface culling that is set to true. If you turn it off, then the backs of the faces will no longer show through.

Second, Your camera/viewport could be inside the mesh a little bit.

Third, your normals could be off in some way. Go into edit mode, select everything, and "recalculate normals." you can find this command by pressing the space bar and typing 'normal' until the command comes up (if you are using 2.79 controls) or by pressing f3 (if you are using 2.8 controls)